Protection device for transformers operating in parallel



R. PENSIS Dec. 29, 1964 PROTECTION DEVICE FOR TRANSFORMERS OPERATING IN PARALLEL 2 Sheets-Sheet 1 Filed Nov. 1, 1961 INVf/VTUA Kern" PINS/5 ar.

ATTORNEYS R. PENSlS Dec 29, 1964 PROTECTION DEV-ICE FOR TRANSFORMERS OPERATING IN PARALLEL Filed NOV- 1. 1961 2 Sheets-Sheet 2 nvwwrok R00 0' Pf/VS/S Fig 2 United States Patent The present invention pertains to a protection device for transformers operating in parallel.

When operation in parallel is contemplated on two or more transformers, each provided with an on-load tap changer, certain precautionary measures must be taken to ensure that the tap changers are on the same voltage step at all times, failing which a circulating current would be established between the secondary windings of the transformers. To obtain this, it is necessary to provide the tapchangers with auxiliary switches having the same number of contacts and to connect all said switches in series in such a way that if one is not in the same position as the others, the circuit would be broken, which would cause actuation of the safety devices. All transformers must, of course, be identical. Under these conditions, only one voltage regulator is used which is connected to the bus-bars common to all transformers and this regulator acts simultaneously on all tap changers.

If several bus-bar sets are used, each is provided with a separate voltage regulator.

The problem becomes more complex where there are several transformers of different types provided with tap changers having voltage steps that are different in both number and size.

In such a case, and according to a known device, each of the transformers is equipped with a voltage regulator acting on its tap changer and a voltage is formed which is substantially proportional to the exchange current between the transformers. To this voltage is assigned a maximum predetermined value. The regulator then interferes to correct the exchange current by acting on the tap changer in the intended direction corresponding to the sign of the variation relative to the predetermined value and with an amplitude proportional to the said variation.

This device is effective but, because of the inertia of the regulators and of the tap-changers associated therewith, if a transformer is connected to the bus-bars before ensuring that its tap changer is in the condition which corresponds to the voltage of the bus-bars and should the position of the said tap changer be too'far from the latter, an appreciable exchange current flows which the tap changer cannot carry without inconvenience.

The present invention makes it possible to obviate the consequences of such a situation by causing the immediate disconnection of the transformer which has been connected to the bus-bars under the above-mentioned conditions.

The invention is characterized in that the circuits supplying the voltage representing the exchange current of each transformer are joined in parallel on a common line, each through the medium of a relay sensitive to the intensity and direction of the current and which actuates contacts causing the disconnection of the transformer in predetermined conditions of intensity and direction of the exchange current.

The invention will be best understood by the description that follows having reference to the appended drawings showing a non-limitative embodiment of the inven tion and wherein:

FIG. 1 is a schematic view of four transformers joined in parallel in accordance with teaching of the invention;

FIG. 2 is a vector diagram relative to the transformers;

FIG. 3 is a schematic representation of a discriminator which can be used in the circuit of FIG. 1.

In FIG. 1, the four transformers T T T T are fed by a set of primary bars P through breakers a a a a and wherein the secondaries discharge in parallel through breakers b b b h, on a set of secondary bars Transformers T T T T are of different construction and have primaries each provided with an on-load tap changer. Voltage regulators R R R R act on the respective tap changers of the four transformers. The voltage to be regulated is supplied to the regulatorsby the potential transformers E E E E To its action is superimposed that of discriminators D D D D which supply a DC. voltage substantially proportional to the reactive current of the transformers and the polarity of which has one direction or the other depending on whether the reactive current is supplied or absorbed by the transformer under consideration. These discrimina tors are set, on the one hand, by the voltage of the potential transformers and, on the other hand, by current transformers I l 1 I The output voltage of the discriminators is applied across the regulators through the medium of calibrating devices, for instance, potentiometers P P P R, which allow its control.

Referring now to REG. 2, if it is assumed that transformer T supplies an exchange current 1 to the other transformers, this current will distribute itself amongst the latter transformers in currents J J J For the sake of simplicity, it is assumed that these currents are completely reactive. Vectors U indicate the bus-bar voltage.

These currents compound with the utilization currents J1, J2, J3, J4 to form total currents 1T1, 1T2, 1T3, 1T4. is seen that the reactive current of transformer T has substantially increased on account of the exchange current whereas it has remained practically the same in the others. The regulators R R R R, will act on their respective tap changers in the direction corresponding to the phase of the reactive currents and with an amplitude proportional to the intensity of said currents in such a manner as to reduce the exchange currents to a minimum. This minimum depends on the adjustment of otentiometers 1! P2: P3! P4 Discriminators D D D D are, for instance, of a type shown schematically in FIGURE 3. Resistances R' and R' are equal; and so are R' and R',;. The bus-bar voltage is applied across terminals A and B and a current proportional to the transformer current is made to flow between terminals C and D. Through the action of rectifiers rd rd and I'd3, it is possible to observe that if the current is in phase with the voltage, the mean value of the voltage between E and F is null, whereas, if the current is out-of-phase with respect to the voltage, a DC. component appears between these terminals which is the larger, the greater the phase displacement; the maximum corresponding to a phase displacement of and with a polarity in a direction or the other, depending on whether the current leads or lags.

According to the invention, the discriminators output voltages are used to reproduce in the form of direct currents the picture of the exchange currents between the transformers; these direct currents acting on protection relays.

To this end, according to FIGURE 1, the output voltages of the discriminators, taken across potentiometers P P P P are applied in parallel across a line comprising conductors g and h. In the various connections to this line are inserted relays 1, 2, 3 and 4. The latter are traversed by currents which, in size and direction, substantially represent the exchange currents in the corre- Patented Dec. 29, 1964 sponding transformers and act on the release of the respective breakers b b b b for a minimum predetermined current. In using polarized relays, or other relays sensitive to the direction of current which energizes them, it is possible to obtain selective protection by limiting it to only those transformers the exchange current of which has a definite direction.

The invention is applicable to the case where the transformer is discharged on one or the other of several busbars. It is only necessary to commute the action of relays 1, 2, 3 and 4 on the breakers that are switched on the busbars in service.

It is obvious that certain changes may be carried out to the above described embodiment without departing from the spirit of the invention as defined in the enclosed claims.

I claim:

1. A protection device for parallel connected transformers, comprising: an on-load tap changer controlling each transformer primary; a voltage regulator controlling the action of each tap changer in response to the secondary voltage of the transformer; a discriminator also controlling each voltage regulator by applying thereto a DC. voltage substantially proportional to the reactive current of the transformer secondary; the polarity of said DC. voltage representing the direction of the reactive current of the transformer; the said discriminators having terminals connected in parallel on a common line and other terminals connected in parallel on another common line, each through a relay, each said relay arranged to be sensitive to the direction of the current energizing it; each said relay thereby being adapted to cause disconnection of the transformer from the remaining said transformers under predetermined conditions of intensity and of direction of the exchange current between said transformers.

2. In a power distribution system including a plurality of power transformers connected in parallel to primary and secondary bus-bars by means of individual breakers, each power transformer having an on-load tap changer operatively connected to an individual voltage regulator responsive to the secondary voltage of the power transformer and to a DC. voltage, supplied by an individual discriminator, substantially proportional to the secondary reactive current of the corresponding power transformer and having a polarity depending upon the lag or lead of said reactive current, individual voltage transformers connected to the secondary of the respective power transformers for feeding said voltage regulators and said discriminators, individual current transformers, connected in series with the secondaries of the respective power transformers, for feeding said discriminators; the combination with said voltage regulators, said discriminators and said breakers of a protective apparatus comprising separate tripping relays, operatively connected to the respective secondary breakers, responsive to a predetermined intensity and a given polarity of their energizing current, a connection common to the coils of said tripping relays, individual connections between said coils and the respective output terminals of said discriminators and a connection common to the other output terminals of said discriminators, said protective apparatus being arranged to cause the tripping of a breaker when the exchange current between the corresponding power transformer and the secondary bus-bars exceeds a predetermined value in a given direction.

References fitted by the Examiner UNITED STATES PATENTS 2,732,507 1/56 Stineman 3l7-26 X SAMUEL BERNSTEIN, Primary Examiner. 

1. A PROTECTION DEVICE FOR PARALLEL CONNECTED TRANSFORMERS, COMPRISING: AN ON-LOAD TAP CHANGER CONTROLLING EACH TRANSFORMER PRIMARY; A VOLTAGE REGULAR CONTROLLING THE ACTION OF EACH TAP CHANGER IN RESPONSE TO THE SECONDARY VOLTAGE OF THE TRANSFORMER; A DISCRIMINATOR ALSO CONTROLLING EACH VOLTAGE REGULATOR BY APPLYING THERETO A D.C. VOLTAGE SUBSTANTIALLY PROPORTIONAL TO THE REACTIVE CURRENT OF THE TRANSFORMER SECONDARY; THE POLARITY OF SAID D.C. VOLTAGE REPRESENTING THE DIRECTION OF THE REACTIVE CURRENT OF THE TRANSFORMER; THE SAID DISCRIMINATORS HAVING TERMINALS CONNECTED IN PARALLEL ON A COMMON LINE AND OTHER TERMINALS 